1. Field of the Invention
This invention relates to a butterfly valve comprising a valve housing including a cylindrical fluid passage, a disc-shaped valve body rotatable about an axis vertical or substantially vertical to the central axis of the fluid passage, and a drive mechanism to turn and lock the valve body, wherein a hard annular seat provided in the housing is adapted to contact a hard annular rim defined on the periphery of the valve body over the entire circumference thereof when the valve body is vertical or substantially vertical to the axis of the fluid passage.
2. Description of the Prior Art
In a valve of this kind, both the seat and the rim are of rigid material such as metal, synthetic resin and hard rubber, which offers an advantage of their being invulnerable to the wearing action of sand and the like contained in fluids, hence long life. Further, since the valve body assumes a position normal to the axis of the fluid passage to close it, the valve body has the shape of a disc, which means an advantage of easiness in manufacture. But, on the other hand, the seat and the rim must have surfaces finished with high precision for tight sealing purposes, thus posing a problem of manufacture in respect of surface finish.
As shown in FIG. 8, the conventional seat 17 has a contacting surface that meets the side surface E of a cone. Generally, because of the functional nature of cutting machines, it is difficult to attain precision in finishing objects in the shape of the side surface of a cone. Thus, improvement on the seat construction is desired for easiness of manufacture and for higher precision in finishing work.